L-band (3.5 mu m) IR-excess in massive star formation - II. RCW 57/NGC 3576

Abstract

Context. We present a JHK(s)L survey of the massive star forming region RCW 57 (NGC 3576) based on L-band data at 3.5 mu m taken with SPIREX ( South Pole Infrared Explorer), and 2MASS JHK(s) data at 1.25-2.2 mu m. This is the second of two papers, the first one concerning a similar JHK(s)L survey of 30 Doradus. Aims. Colour-colour and colour-magnitude diagrams are used to detect sources with infrared excess. This excess emission is interpreted as coming from circumstellar disks, and hence gives the cluster disk fraction (CDF). Based on the CDF and the age of RCW 57, it is possible to draw conclusions on the formation and early evolution of massive stars. Methods. The infrared excess is detected by comparing the locations of sources in JHKsL colour-colour and L vs. (K-s - L) colour - magnitude diagrams to the reddening band due to interstellar extinction. Results. A total of 251 sources were detected. More than 50% of the 209 sources included in the diagrams have an infrared excess. Conclusions. Comparison with other JHKsL surveys, including the results on 30 Doradus from the first paper, support a very high initial disk fraction (> 80%) even for massive stars, although there is an indication of a possible faster evolution of circumstellar disks around high mass stars. 33 sources only found in the L-band indicate the presence of heavily embedded, massive Class I protostars. We also report the detection of diffuse PAHs emission throughout the RCW 57 region.